Baby sitting my baby sister was one of the worst chores I have ever done. This is because this was a big responsibility to handle at a younger age. It is difficult to take care of a young baby whom you cannot communicate to.
Cutting the grass with a push mower... The problem was I had to push the mower about 3 miles with five gallons of gas and I had to mow three sandy ballfields once a week for my summer break. I was about 11 or 12.
The biggest and worst chore I have ever done is removing mildew off my house and then painting the whole house. It took me over three weeks on and off just to finish. It was a big chore but, I am glad I accomplished it, I did get help though.